Monthly Climate Statement for August 2012

It is considered that the probability of above-median rainfall for the next three-month period (August to October) is near-normal to slightly higher than normal (50 to 70 per cent) for most of Queensland. For the coming summer (November to March), the long-range experimental Seasonal Pacific Ocean Temperature Analysis version one (SPOTA-1) scheme currently indicates a slightly higher than normal probability of above-median rainfall across Queensland.Read more (PDF, 217K, last updated 04:35PM, 20 August 2012)*

This assessment is based on the current and projected state of the El Niño-Southern Oscillation (ENSO) phenomenon and on factors which alter the impact of ENSO on Queensland rainfall (e.g. the Pacific Decadal Oscillation (PDO)). The outlook for the August to October period is based on an historical analysis of Queensland rainfall and the SOI. The long-lead outlook for summer is based on the current sea-surface temperature pattern in the extra-tropical Pacific.

As at 1 August 2012, it is noted that sea-surface temperature anomalies in the central equatorial Pacific are nearing El Niño thresholds. Approximately 80 per cent of international global climate models, and most models surveyed by the BoM (‘ENSO Wrap-Up’ (PDF)* July 31), indicate the possibility of an El Niño event developing before summer.

In addition:

The SOI, a key atmospheric measure of ENSO, rose from a quite negative value in June (-10.2) to an average value (+0.1) in July. The Department of Science, Information Technology, Innovation and the Arts (DSITIA) assessment of rainfall probabilities for August to October is based on this rapid rise in the SOI.

Further warming of central Pacific sea-surface temperatures, coupled with quite negative SOI values over the coming months, would be required to confirm the development of an El Niño pattern.

The SOI and sea-surface temperatures will be closely monitored over coming months when ENSO conditions tend to ‘lock in’ and become a more reliable indicator of rainfall for the season ahead. However, it should be noted that a cool phase of the PDO, as currently exists, tends to moderate the impact of El Niño on spring and summer rainfall. The outlook for spring and summer rainfall may change and will be updated monthly.

Rainfall outlook

There are various approaches used to provide rainfall outlooks. These approaches tend to differ in terms of the components of the climate system that are considered and, for this reason, each approach may convey a different outlook, particularly for specific locations.

the experimental SPOTA-1 scheme, which integrates sea-surface temperature information, including indices of ENSO and the PDO; and

the SOI phase scheme, which relies solely on the SOI, an atmospheric measure of ENSO.

The experimental SPOTA-1 scheme provides long-lead probabilities of summer (November to March) rainfall for Queensland from mid-April through to mid-November each year.

An updated assessment of rainfall probabilities for the upcoming 2012/13 summer, based on the experimental SPOTA-1 scheme, indicates a slightly higher than normal probability of above-median rainfall for much of Queensland. This assessment is based, in part, on an index of March sea-surface temperature anomalies which reflect the current ‘cool’ state of the PDO. This latest assessment, which also takes into account a monthly ENSO index, will be revised from September through to November this year.

DSITIA’s SOI phase scheme provides probabilities of rainfall for the coming three-month season based on SOI values over the previous two months. The SOI phase scheme currently indicates that the probability of above-median rainfall across most of Queensland is 50 to 70 per cent for the next three-month period (August to October). This analysis is based on the SOI being in a ‘Rapidly Rising’ phase at the end of July, as discussed further in the Australia's Variable Rainfall poster (PDF, 9.3M, last updated 03:09PM, 29 July 2016)*, or the Department's archive of historical rainfall maps).

Each of the above schemes may have their own particular following. Although such schemes cannot provide outlooks with absolute certainty, users of the information who follow a skilful scheme should benefit from doing so in the long-term. Users should consider the historical track record of any scheme, and such historical information is becoming increasingly available.

DSITIA's Long Paddock website provides the historical archive of SPOTA-1 reports and past commentaries on the SOI phase scheme. Users should also consider the wide range of information available each month describing the current state of the ocean/climate system, for example the 'ENSO Wrap-Up' (PDF)*.

ENSO influences other climate variables apart from rainfall (e.g. temperature, pan evaporation and vapour pressure). This means that the impact of ENSO on crop or pasture growth can be stronger than on rainfall alone. The impact of ENSO on pasture growth is also dependent upon current pasture condition and soil water status. DSITIA’s AussieGRASS model takes these factors into account in producing seasonal pasture growth probabilities.